1. Field of the Invention
This invention generally relates to semiconductor devices on a wafer structure and, more particularly, to interconnection of the semiconductor devices on the wafer structure.
2. Discussion of the Related Art
By using a local interconnect technique, a 25% reduction in CMOS SRAM cell size has been demonstrated as discussed in O. Kudoh, et al., "A new full CMOS SRAM cell structure," IEDM 1984, pp. 67-70, while a 64 Mb MOS DRAM with a 0.4 .mu.m design rule has been reported as disclosed in M. Sakao, et al., "A capacitor-over-bit-line cell with a hemispherical-grain storage node for 64 Mb DRAM," IEDM 1991, pp. 655-658. However, from past experience, there are many challenges presented when implementing such local interconnect techniques.
For instance, it is especially difficult to provide local interconnects when devices are densely packed or include rough topography. Some examples include: (1) poor etch selectivity between the interconnect material and the active device material, such as source/drain areas of an FET, can cause severe etch damage to the devices when overetch is applied in order to eliminate unwanted conductive sidewalls; (2) the overall wiring resistance is not satisfactory because a thin interconnect is used in order to gain better control on patterning and also because the effective wiring length is longer than intended due to the existing topography, (3) local wiring aggravates the existing topography, and (4) finally, further scaling is limited by ground rules, e.g., the minimum allowable distance between a local interconnect and a polysilicon gate.
In U.S. Pat. No. 5,010,386, an insulator separated vertical CMOS structure is disclosed. Transistor device layers are built above and below a buried oxide layer. Subsequently, CMOS inverters are formed by trench etching through a transistor stack and connecting desired transistors. A disadvantage of the '386 structure is that the forming of a desired structure requires numerous process steps including exposure of the lower devices to additional high temperature processing. Additionally, the positioning and accuracy of an interconnection between devices is subject to the tolerances of a trench fill material.
In U.S. Pat. No. 4,829,018, a wafer bonded epitaxial multilevel integrated circuit is disclosed. Circuit devices in the epitaxial layers are interconnected by forming conductive vias between the epitaxial layers. Such interconnections are non-planar and require complex lithography steps. This is undesirable since it is not well suited for manufacturability.
It would thus be desirable to provide an interconnect structure which results in no damage to device areas and no added topography which also eliminates sidewall formation, while providing the desired interconnections. It would further be desirable to provide a simplified method of interconnecting desired devices on a substrate.